It is known in the art that the wet process mode of producing a precipitated silica involves (1) forming a gel, preferably by mixing sodium silicate and sulfuric acid in a solution containing a salt, and (2) breaking the gel, thereby obtaining a precipitated suspension of silica. The suspension is filtered, forming a filter cake, washed free of occluded salts, and slaked, forming a slurry by adding water, which makes the filter cake sufficiently fluid to be spray dried. However, spraying also requires the use of an atomizable suspension, which is difficult to produce.
Attempts have been made to obtain such an atomizable suspension, but each process has either economical or practical drawbacks. For example, in one process, the procedure can only be used with dry extracts and a filter cake low in silica content, but this process requires high levels of drying energy. Mechanical slaking systems, which reduce the viscosity of the filter cake, also require a substantial amount of energy.
Other attempts to produce an atomizable suspension include the product disclosed in European Pat. No. 18,666, which is produced by spray drying a suspension with a pH greater than 4, preferably between 4.5 and 6. However, the stability of a silica suspension decreases as the pH value moves away from the isoelectric point, which for most silica suspensions is at a pH of greater than 3.5-4, therefore, the suspension cannot be maintained at the proper pH without agitation, a considerable efficiency constraint. Thus, the process has limited industrial use. Similarly, U.S. Pat. No. 3,208,823 discloses a process of acidifying a filter cake, but that process is not successful when neutral pH conditions are required, such as for use in rubber.
The present invention addresses these difficulties by producing a stable, easily atomizable suspension with improved rheological properties and a high solids content. The process of the present invention concerns stable suspensions of precipitated silica, with a relatively high solids content, and a pH value greater than 3.5-4.